Support providing separate API keys for strong and weak models

[githubcustomerserviceistrash]

Currently the controller only takes a scalar value API key argument. If I want to override the default API key for a specific call both models need to be from the same provider. I'd like to be able to provide a dict of { litellm_provider: key } to the api_key argument to enable precise key control.

[iojw]

This is a great suggestion, will put on the to-do list (but definitely welcome any contributions).